Fluorescent materials are used for vacuum fluorescent display tubes (VFDs), field emission displays (FEDs), plasma display panels (PDPs), cathode-ray tubes (CRTs), white light-emitting diodes (LEDs), etc. To allow the fluorescent material to emit light in any application, energy for exciting it must be supplied to it. Upon excitation by an excitation source having high energy such as vacuum ultraviolet radiation, ultraviolet radiation, electron radiation, blue light or the like, the fluorescent material gives out visible light rays. A problem with the fluorescent material is, therefore, that its luminance drops as a result of exposure to such an excitation source as mentioned above. To overcome that problem, sialon fluorescent materials have been proposed as those having more limited luminance decreases than do prior art fluorescent materials based on silicates, phosphates, aluminates, and sulfides.
The sialon fluorescent material, for instance, has been prepared by mixing together silicon nitride (Si3N4), aluminum nitride (AlN) and europium oxide (Eu2O3) at a given molar ratio, and then subjecting the resulting mixture to hot-press firing wherein it is held at a temperature of 1,700° C. for 1 hour in nitrogen of 1 atm (0.1 MPa) (for instance, see patent publication 1). α-sialon with activated Eu ions, obtained by this method, has been reported to provide a fluorescent material that is excited by blue light of 450 to 500 nm, giving out yellow light of 550 to 600 nm. In situations where ultraviolet radiation such as one emitted out of an ultraviolet LED or lamp is used for an excitation source, however, a fluorescent material that is efficiently excited at a wavelength of 400 nm or lower is still in demand. Besides, fluorescent materials emitting not only yellow light but also blue light or green light are still in need. There are also demands for materials having ever higher emission luminance.    Patent Publication 1: JP(A)2002-363554